Within the field of air handling it is common to equip an air handling unit with some kind of heat recovery device to recover energy from the heated exhaust air and transfer this energy to the supply air, in case of a heating demand of the supply air. The opposite, to cool down the supply air in case of the exhaust air is colder than the outdoor air, is also a common task of the heat recovery device. Irrespective of the type of heat recovery it is common with some kind of after-treatment of the supply air, for example a heating coil and/or a cooling coil for controlling the supply air temperature to a wanted value, a so called set point, despite the outdoor air temperature.
The state of the art also presents heat pump solutions in connection with air handling units, and these are called for example Heat Pumps, Coolers and the like, depending of if the main function is heating, cooling or heat recovering. Practically, the placement of the heat pump is in connection to or in the air handling unit, and the heat pump comprises a refrigerant circuit with a heat transfer medium/a refrigerant of some kind, an evaporator, a condenser, as well as a compressor and an expansion valve. The heat pump can be designed with a so called reversible refrigerant circuit which means that a multi-way valve can, if necessary, change the flow direction of the refrigerant to the opposite compared to the normal flow direction. In connection to an air handling unit, a so called direct expansion coil is placed—a DX-coil—in the supply air and one DX-coil in the extract air, i.e. in the exhaust air, in the flow direction after the heat exchanger, thus in the air ready to be released into the open air. These DX-coils represents evaporator or condenser, depending on the flow direction in which the refrigerant is pumped in the system. DX-coils typically consist of a number of copper tubes with aluminum fins where the refrigerant passes through the copper tubes and heat is emitted or absorbed through the exchange with the ambient air passing through the DX-coil. The aluminum fins increases the heat transfer surface of the DX-coil.
The general function of the heat pump process is such that the heat content of a medium, such as air, is moved from a location where the heat can be collected, to another location where there is a heat demand and where heat can be emitted. The heat pump process works in that the refrigerant in the system transfer from high pressure to a significant lower pressure when passing an expansion valve, which also means that the temperature of the refrigerant drops radically. The refrigerant then passes the evaporator where it evaporates due to heat absorption from the ambient medium, such as outdoor air, or like in this case the exhaust air/extract air from an air handling unit. The steam then passes the compressor where it is compressed to once again obtain a high pressure, while the temperature increases during compression, and in most cases, compressor heat is recovered and transferred to the refrigerant as well. Then the refrigerant is sent, in the form of heated high pressure steam, on to the condenser, where the high-pressure steam condenses and becomes liquid. During this process, the condenser releases heat, and it is this heat, along with any compressor heat that is used for heating the heating side. Thereafter the refrigerant is sent to the expansion valve, and the heat pump cycle is thus closed. When the circuit is used for cooling in an air handling unit, the process runs simply reversibly, that is, the refrigerant is sent in the other direction in the system, wherein each DX-coil so to say shifts, so that the evaporation takes place instead into the supply air, which is then cooled, while condensation occurs into the exhaust air/extract air.
When these systems operates, a need to defrost of the evaporator arises every now and then, because the evaporator side DX-coil is cooled down rapidly, both because of the evaporation process itself, with very cold refrigerant in the evaporator, and that the evaporator is exposed to cold and often damp air that passes through the evaporator/DX-coil. This applies especially in the heating case, that is, when heat is absorbed from the exhaust air/extract air and transferred to the supply air for heating the same. The result of this is that heavy icing may occur in the evaporator unless defrost. Using the heat pump process in a cooling mode, and in connection with an air handling unit, is relatively common and sometimes also when the unit has a heat exchanger. Due to the above described icing problems there are no known solutions with combinations of heat exchanger and heat pump for heating applications, and with effective and useful defrosting technique, because no one has figured out a good enough defrosting technique of the evaporator. It simply costs too much energy and the supply air temperature fluctuates due to the defrosting sequence (cooling of the supply air during defrost despite heating demand). By that the air handling unit comprises a heat exchanger which, by heating is regulated for maximum heat recovery from exhaust air, it follows that the temperature after heat recovery and before the evaporator is cold. The combination of this, along with cold refrigerant within the evaporator, increases the risk of ice formation on the evaporator and heavy icing can occur as stated. According to conventional methods in most types of heat pumps, the defrosting operation is conducted by that the refrigerant circuit is driven reversibly, by arranging a multi-way valve or the like, in the refrigerant circuit, which valve, at a defrosting demand, turns the refrigerant flow direction, wherein warm refrigerant is sent to the evaporator, instead of the condenser, during an appropriate time to make the ice melt and the DX-coil is thereby heated from the inside with the refrigerant. The control of this constitutes either a timer controlled defrosting sequence where the sequence is repeated according to preset intervals or either by a frost guard indicating when icing occurs, where after the defrosting sequence starts. In these cases the defrosting is conducted according to on/off-principal, i.e. either defrosting in progress or not in progress, and the defrosting in progress is conducted according to a certain time or until the system indicates that the ice formation is gone.
In other heat pump applications, like a conventional air/air heat pump or a cooler, the placing of the evaporator is separate out in the outdoor air, on a roof or on a wall or the like. The evaporator in these applications is often equipped with a fan which blows or sucks air through the evaporator. During defrosting the refrigerant circuit is reversed at the same time as the fan is shut off so as not to cause an extension of the defrost cycle, if the ambient air is cold. Within the field of ventilation the conditions is somewhat different when the air handling fans must run continuously, even during defrosting of the evaporator, due to ventilation requirements. This effect the defrosting cycle time, which is extended, and the building, which is ventilated by means of air handling unit, must be ventilated during the time of defrosting. The longer the defrosting cycle time takes the more energy is lost, along with that the supply air temperature gets colder, and this is a problem that led to the use of heat pump solutions for heating of the supply air is rare. Cooling by heat pump is, as mentioned above, however, relatively common because then there is usually not the same problem, as the evaporator is set to never get colder than, say, +15° C., as you very rarely can allow a supply air temperature colder than this.
When an air handling unit with a heat pump solution is run in heating mode, i.e. during the cold season of the year, occurs as told, continuously the need of defrosting during operation. On the supply air side in the field of ventilation, there is another disadvantage as mentioned above, namely, the temperature demand on the supply air, which is not a problem in other heat pumps arranged for heating, where the ventilation demand is not at hand. By reversible operation as defrosting technique, the respective DX-coil changes, at least eventually, from evaporator side to condenser side and vice versa, so to speak. The DX-coil placed in the supply air after the heat exchanger, is normally in heating case the final heating unit, for heating the supply air to the set value, after the heat exchanger has transferred heat to the supply air. The DX-coil of the supply air side constitutes thus condenser in the heating case, but at defrosting of the evaporator (in extract air) by reversible operation, the supply air DX-coil becomes the cold side. While ventilation cannot be shut off it means that the supply air eventually cools down, despite the need of heat. The supply air temperature cannot be kept constant in other way than installing an extra heating battery in the supply air. It is desirable that the time of defrosting is minimized, precisely because of these unique disadvantages within the field of ventilation.
The problem with prior art is that the defrosting sequence is considerably time consuming, while the controlled defrosting takes place unilaterally, from the inside of the evaporator by heating the refrigerant. Furthermore, the heat recovery operates parallel to defrosting, which itself cools the air passing the evaporator and thereby extends the defrosting time, and that both supply air fans and exhaust air fans have to run continuously.